Memory cell

ABSTRACT

Methods, and circuits, are disclosed for operating a programmable memory device. One method embodiment includes storing a value as a state in a first memory cell and as a complementary state in a second memory cell. Such a method further includes determining the state of the first memory cell using a first self-biased sensing circuit and the complementary state of the second memory cell using a second self-biased sensing circuit, and comparing in a differential manner an indication of the state of the first memory cell to a reference indication of the complementary state of the second memory cell to determine the value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.12/053,236 filed Mar. 21, 2008, now U.S. Pat. No. 7,813,167 issued Oct.12, 2010, the specification of which is incorporated by referenceherein.

TECHNICAL FIELD

The invention relates to semiconductors and semiconductor memorydevices. More particularly, in one or more embodiments the inventionrelates to sensing programmable resistance memory cells used as fuses inmemory devices.

BACKGROUND

Memory devices are typically provided as internal, semiconductor,integrated circuits in a wide range of processing applications, such ascomputers or other electronic devices. There are many different types ofmemory devices including random-access memory (RAM), read only memory(ROM), dynamic random access memory (DRAM), synchronous dynamic randomaccess memory (SDRAM), flash memory, “phase-change” random access memory(PCRAM), and resistance random access memory (RRAM), among others.Memory devices can be comprised of an array arrangement of memory cells.

A memory cell stores digital information in a structure that can berapidly switched between more than one readily discernable state. Somememory cells are based on the presence or absence of electrical chargecontained in a region of the cell. By retaining its charge, the memorycell retains its stored data. Some memory cell structures inherentlyleak charge, and must be continually powered to refresh the storedcharge.

Non-volatile memory however, does not require electrical power to retaincharge information. For example, flash memory typically has a “floatingleakage. Thus, power is required only to change the stored information,e.g., write-to (store charge), read-from (determine if charge ispresent), or erase (remove charge) data. The non-volatility of storeddata in flash memory is advantageous in portable electronicapplications. Uses for non-volatile memory include personal computers,personal digital assistants (PDAs), digital cameras, and cellulartelephones. Program code and system data, such as a basic input/outputsystem (BIOS) used in personal computer systems (among others), aretypically stored in non-volatile memory devices.

“Phase change,” memory cells use detectable changes in physicalstructure of the memory cell material to define various states, e.g.,resistance changes associated with different molecular structures of thememory cell material. The various states can be associated with digitalinformation. The physical layout of a PCRAM or RRAM memory cell within amemory device array may be arranged similarly to a DRAM memory cell;however, the capacitor of the DRAM cell is replaced by a material havingdetectable “phase change” characteristics, e.g., resistance states.

Memory cells in an array architecture can be programmed to a desiredresistance state. A single “phase change” memory cell may have more thantwo discernable “phase” states, each “phase” state having acorresponding different resistance state, and thus may store more thantwo data values, e.g., digits. Such memory cells may be referred to asmulti state memory cells, multidigit cells, or multilevel cells (MLCs).MLCs can allow the manufacture of higher density memories withoutincreasing the number of memory cells since each cell can represent morethan one binary digit, e.g., more than one bit. MLCs having more thanone programmed state, e.g., a memory cell capable of representing twodigits can have four programmed states, a cell capable of representingthree digits can have eight program states, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a prior art example “phase change”memory cell structure.

FIG. 2 illustrates a temperature-time relationship during programmingfor a prior art “phase change” rewritable memory cell.

FIG. 3 illustrates current-voltage characteristics for a prior art“phase change” rewritable memory cell in both the amorphous (“highresistance”), and crystalline (“low resistance”) states.

FIG. 4A illustrates a portion of a schematic of a memory array includingdiode access devices for a “phase change” rewritable memory cell inaccordance with one or more embodiments of the present disclosure.

FIG. 4B illustrates a portion of a schematic of a memory array includingmetal oxide semiconductor field effect transistor (MOSFET) accessdevices for a “phase change” rewritable memory cell in accordance withone or more embodiments of the present disclosure.

FIG. 4C illustrates a portion of a schematic of a memory array includingbipolar junction transistor (BJT) access devices for a “phase change”rewritable memory cell in accordance with one or more embodiments of thepresent disclosure.

FIG. 4D illustrates a portion of a schematic of a memory array havingvariable, e.g., programmable, resistance elements, and sensingcircuitry.

FIG. 5 illustrates a schematic of a prior art sensing amplifier circuit.

FIG. 6 illustrates a schematic of a self-biased sensing circuit inaccordance with one or more embodiments of the present disclosure.

FIG. 7 illustrates a schematic of a 1^(st) and 2^(nd) stage of a fusesense circuit, in accordance with one or more embodiments of the presentdisclosure.

FIG. 8A illustrates a schematic of a memory cell in a firstconfiguration as a fuse with a “phase change” structure and an accesstransistor, in accordance with one or more embodiments of the presentdisclosure.

FIG. 8B illustrates a schematic of a memory cell in a secondconfiguration as a fuse with a “phase change” structure and an accesstransistor, in accordance with one or more embodiments of the presentdisclosure.

DETAILED DESCRIPTION

The present disclosure includes methods, and circuits, for operating aprogrammable memory device. One method embodiment includes storing avalue as a state in a first memory cell and as a complementary state ina second memory cell. Such a method further includes determining thestate of the first memory cell using a first self-biased sensing circuitand the complementary state of the second memory cell using a secondself-biased sensing circuit, and comparing in a differential manner anindication of the state of the first memory cell to a referenceindication of the complementary state of the second memory cell todetermine the value.

In the following detailed description of the present disclosure,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration how one or more embodimentsof the disclosure may be practiced. These embodiments are described insufficient detail to enable those of ordinary skill in the art topractice the embodiments of this disclosure, and it is to be understoodthat other embodiments may be utilized and that process, electrical,and/or structural changes may be made without departing from the scopeof the present disclosure.

FIG. 1 is a diagram illustrating a prior art example of a “phase change”memory cell structure. One skilled in the relevant art will recognizethe “phase change” memory cell shown in FIG. 1 as being a diode accesscell using a vertical resistive electrode for beating a chalcogenideprogrammable volume.

Memory cell 100 includes a diode 102, formed by the junctions betweenthe p− layer 104, n layer 106, and p+ layer 108. Formed atop the diode102 is a resistive electrode 110, across which heat is generated bycurrent flow therethrough. A chalcogenide material layer 112 is formedbetween the resistive electrode 110 and a metal interconnect layer 114.

Current passing through the diode 102, through the resistive electrode110, and to the metal interconnect 114, causes the resistive electrode110 to generate sufficient heat to change the “phase” of a portion ofthe chalcogenide 112 from a crystalline “phase” to an amorphous “phase.”The portion of the chalcogenide 112 which incurs the “phase change” isknown as the programmable volume 116. The crystalline “phase” of thechalcogenide 112 has different electrical resistance characteristicsthan the amorphous “phase.” Thus, the programmable volume 116 hasdifferent resistance states corresponding to different proportions ofcrystalline and amorphous “phases” into which the material isprogrammed.

One skilled in the art will appreciate that a memory cell mayalternatively be fabricated using a lateral structure, and may include acomplementary metal oxide semiconductor (CMOS) transistor, or other typetransistor, access device. Operation of such a “phase change” device issimilar to the memory cell 100 shown in FIG. 1, in that current flowcauses the “phase change” material to change states, e.g., from a morecrystalline chalcogenide to a more amorphous chalcogenide.

FIG. 2 illustrates a temperature time relationship during programmingfor a prior art “phase change” rewritable memory cell. “Phase change”materials, e.g., Germanium-Antimony-Telluride (GST) or otherchalcogenide materials, can be used to create variable resistance memorydevices. Chalcogenide materials can include compounds of sulfides,sclenides, and tellurides, among others. A “phase change” material caninclude a number of Germanium-Antimony-Tellurium materials, e.g.,Ge—Sb—Te such as Ge₂Sb₂Te₅, Ge₁Sb₂Te₄, Ge₁Sb₄Te₇, etc. The hyphenatedchemical composition notation, as used herein, indicates the elementsincluded in a particular mixture or compound, and is intended torepresent all stoichiometries involving the indicated elements.

Other “phase change” materials can include GeTe, In—Se, Sb₂Te₃, GaSb,InSb, As—Te, and Al—Te. Additional “phase change” materials can includeTe—Ge—As, In—Sb—Te, Te—Sn—Se, Ge—Se—Ga, Bi—Se—Sb, Ga—Se—Te, Sn—Sb—Te,and In—Sb—Ge. Some “phase change” memories may include a “phase change”material such as Te—Ge—Sb—S, Te—Ge—Sn—O, Te—Ge—Sn—Au, Pd—Te—Ge—Sn,In—Se—Ti—Co, Ge—Sb—Te—Pd, Ge—Sb—Te—Co, Ge—Sb—Se—Te, Ge—Sn—Sb—Te,Ge—Te—Sn—Pd, and Ge—Te—Sn—Pt, among others. Embodiments of the presentinvention are not limited to the above-listed mixtures and compounds,and may include impurities and the addition of other elements as well.

As shown in FIG. 1, a diode, metal oxide semiconductor field effecttransistor (MOSFET), or bipolar junction transistor (BJT), can beconnected in series with the “phase change” material as an accessdevice. Embodiments of the present invention are not limited to theabove-mentioned type switching devices, and may employ any suitablecontrollable switch technology as an access device.

The storage mechanism of the “phase change” material is the controllableamorphous-crystalline structural changes which can be made to occur inthe material. Accompanying the structural changes to the materials areobservable characteristic changes. For example, the resistance of thematerial changes as the structure of the material changes betweencrystalline and amorphous structure. Generally, the changes to theamorphous-crystalline structure of material is referred to as a “phasechange”; however, embodiments of the present invention are not solimited. For example, resistance changes to a material used inaccordance with embodiments of the present invention may be controlledby affecting other properties of the material, e.g., without changingthe molecular organization, or “phase,” of the material, such asprocessing to change conductivity of a material mixture by rearrangingstructures of one or more constituents of the mixture, causing chemicalreactions involving the mixture constituents, causing micro or macroseparation of certain mixture constituents, among others.

The materials are known as “phase change” materials because glassymaterials are produced by rapidly super cooling a liquid below itsmelting point to a temperature at which atomic motion necessary forcrystallization cannot readily occur. Chalcogenide alloys are typicallygood glass-formers. In order to crystallize as amorphous region of achalcogenide material, the material can be heated to a temperaturesomewhat below the melting point and held at this temperature for a timesufficient to allow crystallization to occur.

When a programmable volume in the crystalline state, the material actslike a linear resister, with a resistance of approximately 5 K-ohms orless according to one embodiment. When the programmable volume is in theamorphous state, a “phase change” memory cell has a characteristicthreshold voltage, around 0.8V according to one embodiment. When avoltage below the threshold is applied to the programmed resistivematerial, the resistance is extremely high, e.g., 50 K-ohms or more. Thereader will appreciate that in certain embodiments, there may only be afactor of 10, i.e., one or more order of magnitude, difference betweenthe resistance values of the binary states. When a voltage above thethreshold is applied to the programmed resistive material, theresistance is similar to the value in the crystalline state.

A single level cell (SLC) can be programmed to a generally moreamorphous (reset) state or a generally more crystalline (set) state.Such reset and/or set states may be taken to correspond to a binary 0and/or 1 for SIX devices. A reset pulse can include a relatively highcurrent pulse applied to the cell for a relatively short period of time.The current applied, to the cell can be quickly reduced after the “phasechange” material “melts” allowing it to cool quickly into a moreamorphous state where atomic motion that can allow crystallizationgenerally occurs to a lesser degree due, at least in part, to relativelyrapid cooling of the material. Conversely, a set pulse can include arelatively lower current pulse applied to the cell for a relativelylonger period of time with a slower quenching speed, e.g., the currentmay be more slowly reduced allowing, the “phase change” material greatertime to cool. Accordingly, the material may crystallize to a greaterdegree than after the reset pulse. Thus, the “phase change” materialscan be made to have a greater resistivity associated with a moreamorphous state and a lesser resistivity associated with a morecrystalline state.

FIG. 2 shows one example of an amorphizing (reset) pulse 220 over timet1, and a crystallizing (set) pulse 222 over time t2. The exampleembodiment of the amorphizing (reset) pulse 220 involves raising thetemperature of the programmable volume, to a temperature Ta (to scramblesome portion of internal crystalline structures) and rapidly cooling theprogrammable volume over a short time period, i.e., t1, such that theprogrammable volume cannot re-form some portion of the internalcrystalline structures. The example embodiment of the crystallizing(set) pulse 222 shown in FIG. 2 involves raising the temperature of theprogrammable volume above a temperature Tx and keeping it there for asufficient period of time, i.e., t2, such that the structure of theprogrammable volume has time to organize into the crystalline structure.

FIG. 3 illustrates current-voltage characteristics for a prior art“phase change” rewritable memory cell in both the amorphous (“highresistance”), and crystalline (“low resistance”) states. Thecurrent-voltage characteristics are shown for “phase change” materialbeing in a crystalline (set) state 324, and for “phase change” materialbeing in an amorphous (reset) state 326. As shown in FIG. 3, the “phasechange” material exhibits different resistive properties at low appliedvoltages (as indicated by the associated currents). These differentresistive characteristics can be exploited at low voltages to ascertainwhich phase the programmable volume is in at any given time. Note thatthe current is near zero for programmable volumes in the amorphous(reset) state 326 until device voltages reach a threshold value, Vth334.

A road voltage regime 328 can be established for a given chalcogenide atlower device voltages. Applying voltage and measuring current, orapplying current and measuring voltage in this region indicates whetherthe programmable volume is in an amorphous (reset) state or acrystalline (set) state. In this manner, the programmable volume is readfor the binary information contained by the resistance state in which itis presently in. In this manner, a bias current may be applied to thememory cell having a “phase change” element, with the resulting voltagebeing examined to determine whether the memory cell is in an amorphousstate corresponding to one resistance state associated with one binaryvalue, or a crystalline state corresponding to another resistance stateassociated with another binary value.

The resistive characteristics of the two material phases shown in FIG.3, e.g., crystalline (set) state 324 and amorphous (reset) state 326,effectively coincide within a set current regime 330, and with anassociated reset current regime 332, as shown in FIG. 3.

“Phase change” memory uses programmable elements (which are referred tohereinafter as “fuses”) for redundancy and adjusting, e.g., trimming,setting, programming, etc., of reference voltages, reference currentand/or timing. When in the amorphous state, a “phase change” memory cellhas a characteristic threshold voltage, e.g., Vth 334. When in thecrystalline state, a “phase change” memory cell acts similar to a linearresistor. As described above, during a road, a current is applied to thememory cell and a voltage is generated across the memory cell.

For a memory cell in a high resistance state, programming to a lowresistance state needs a voltage pulse exceeding Vth, which suppliessufficient dynamic ON-state current to achieve the temperature necessaryfor crystallization. Programming a low resistance memory cell into ahigh resistance needs only sufficient current to melt the chalcogenide,with a quench back to the amorphous state. In read mode, verifying thecell resistance is accomplished at a voltage less than Vth, typically upto ˜0.4 V, which results in a current less than the minimum to achievethe low resistance state programming. Verification can be done bysupplying a voltage and sensing current, or by supplying a read (bias)current and measuring the voltage.

Care should be exercised when reading a “phase change” memory cell sothat the contents are not corrupted during the read, e.g., too muchcurrent can generate heat to cause an inadvertent “phase change.” Themaximum voltage allowed across the memory cell during a read can besubstantially lower than the threshold voltage Vth of the “phase change”material, or data corruption could occur. Limiting the maximum voltageallowed across a memory cell is discussed further below.

The physical layout of a “phase change” memory device, e.g., PCRAM, mayresemble that of a DRAM memory device where the capacitor of the DRAMcell is replaced by a structure of “phase change” material,Germanium-Antimony-Telluride (GST) or other chalcogenide materials. Anaccess device, such as a diode, metal oxide semiconductor field effecttransistor (MOSFET), or bipolar junction transistor (BJT) can beconnected in series with the “phase change” material in a manner thatpermits selection of individual memory cells for operation thereof.

The physical layout of a resistive RAM (RRAM) device may have memorycells including a variable resistor thin film, e.g., a colossalmagnetoresistive material. The thin film can be connected to accessdevices such as diodes, field effect transistors (FETs), BJTs, or otherelectronic switching devices.

FIG. 4A illustrates a portion of a schematic of a memory array includingdiode access devices for a phase-change rewritable memory cell inaccordance with one or more embodiments of the present disclosure. Thememory array portion 401 includes a number of “phase change” memoryelements 440 (indicated with a delta phase symbol), each having aresistance that varies with “phase” of the programmable resistancematerial, e.g., a “phase change” element or a variable resistance orswitchable-resistive element. Each memory element 440A is coupled to asense line, e.g., 442A (BL0), 444A (BL1), 446A (BL2), and to a selectline, e.g., 448A (WL0), 450A (WL1), 452A (WL2), by an access diode 441A.

As one skilled in the art will appreciate, to access a particular memorycell, a corresponding select line, e.g., WL1, can be biased at a firstvoltage, e.g., 0V, while surrounding select lines, e.g., WL0 and WL2,are biased at a second voltage, e.g., 3V. A sense line, e.g., BL1,corresponding to the particular memory cell can then be biased at afirst voltage, e.g., 1V, while surrounding sense lines, e.g., BL0 andBL2, can be biased at a second voltage, e.g., 0V. In this manner,individual memory cells may be accessed through their correspondingaccess device, e.g., diode 441A, to enable sensing of the memory cell,among other functions.

FIG. 4B illustrates a portion of a schematic of a memory array includingmetal oxide semiconductor field effect transistor (MOSFET) accessdevices for a phase-change rewritable memory cell in accordance with oneor more embodiments of the present disclosure. The memory array portion401B includes a number of “phase change” (with corresponding variableresistance characteristics) memory elements 440B, e.g., a “phase change”element or a resistive switching element. Each memory element 440B iscoupled to a sense, e.g., bit, line, e.g., 442B (BL0), 44413 (BL1),44613 (BL2), and to a select, e.g., word, line, e.g., 448B (WL0), 450B(WL1), 452B (WL2), by a transistor access device 441B, e.g., a metaloxide semiconductor field effect transistor (MOSFET).

As one skilled in the art will appreciate, to access a particular memorycell, a corresponding select line, e.g., WL1, can be biased at a firstvoltage, e.g., 1.8V, while surrounding select lines, e.g., WL0 and WL2,are biased at a second voltage, 0V. A sense line, e.g., BL1,corresponding to the particular memory cell can then be biased at afirst voltage, e.g., 0.3V, while surrounding sense lines, e.g., BL0 andBL2, can be biased at a second voltage, e.g., 0V. In this manner,individual memory cells may be accessed through their correspondingaccess device, e.g., MOSFET 441B, to enable sensing of the memory cell,among other functions.

FIG. 4C illustrates a portion of a schematic of a memory array includingbipolar junction transistor (BJT) access devices for a “phase change”rewritable memory cell in accordance with one or more embodiments of thepresent disclosure. The memory array portion 401C includes a number of“phase change” memory elements 440C, e.g., a “phase change” element or aswitchable resistance or variable resistive element. Each resistancevariable memory element is coupled to a sense line, e.g., 442C (BL0),444C (BL1), 446C (BL2), and to a select line, e.g., 448C (WL0), 450C(WL1), 452C (WL2), by an access transistor, e.g., a bipolar junctiontransistor (BJT). Note that the orientations of the sense and selectlines are shown reversed in FIG. 4C from those shown in FIGS. 4A and 4B.However, accessing a particular individual memory cell is accomplishedin a similar manner as described above for the diode array illustratedin FIG. 4A.

FIG. 4D illustrates a portion of a schematic of a memory array havingvariable, e.g., programmable, resistance elements, andsensing-circuitry. The memory array portion 401D includes a number of“phase change” memory elements 440D (with corresponding variableresistance characteristics). Each memory element 440D includes atransistor access device 441D, e.g., a MOSFET, coupled in series with avariable resistance 445, such as a programmable volume of “phase change”material having associated resistance states which vary with phaseconstituency. One terminal of the transistor access device 441D isconnected to ground, and thus provides a switchable path to ground forcontrolling current flow through the variable resistance 445.

The gate of the transistor access device 441D is coupled to a senseline, e.g., 442D (Cn−1), 444D (Cn), 446D (Cn+1), and one terminal of thevariable resistance 445 is coupled to a select line, e.g., 448D (Rn−1),450D (Rn), 452D (Rn+1). Accessing individual memory cells isaccomplished in the same manner through the MOSFET transistor, as wasdescribed above with respect to FIG. 4B. Select lines correspond to rowsof the array shown in FIG. 4D, also referred to as word lines in someembodiments. Sense lines correspond to columns of the array shown inFIG. 4D, also referred to as bit lines in some embodiments.

A sense circuit, e.g., 447, is coupled to each respective sense line,e.g., 442D (Cn−1), 444D (Cn), and 446D (Cn+1). The sense circuit 447supplies sense current, for example from a current source 443, to amemory element 440D, and compares the sense line voltage (typically) toa reference voltage 453 through amplifier 449 to produce an output 451.Sense line current is sunk through one of a number of controltransistors 455, depending on type of operation, e.g., set, reset, orread.

Although a number of access devices and example operating parameters forprogrammable resistance memory coils have been illustrated and describedin connection with FIGS. 4A-4D, embodiments of the present disclosureare not so limited. Other types and arrangements of access devices,memory array architectures, and operating parameters are contemplated inone or more embodiments of the present invention, as will be understoodby one of ordinary skill in the art.

One skilled in the art will appreciate that the sense circuit 447 needsan accurate reference voltage 453 signal to perform accurate sensing ofmemory cells, e.g., the voltage developed as a result of a bias currentbeing passed through a programmable resistance is compared to thereference voltage corresponding to a particular digital value. Asmentioned, memory, such as “phase change” memory, uses fuses not onlyfor redundancy, but also for adjusting the values of reference voltages.According to one technique, information related to the generation ofsuch reference voltage signals may be stored in a portion of the memory.However, if a reference voltage is required to read the informationstored in memory, which in turn is needed to accurately adjust thereference voltage, then a “chicken and egg,” or circular, problemarises. If errors occur in the reference voltage, subsequent errors inthe interrogation of the other memory cells may follow.

Some portion of the memory cells of a memory device may be used asfuses. For example, “phase change” memory with its variable resistancecan be appropriately programmed to vary resistance in a circuit so as toadjust circuit response as a means of adjusting certain signalsassociated with the memory, including for example, reference voltagemagnitudes.

As discussed above with respect to FIG. 3, care should be exercised whenreading a “phase change” memory cell so that the contents are notcorrupted during the read, such as confining the read voltage applied toa programmable resistance memory cell to the read voltage regionillustrated in FIG. 3. During the reading of a memory cell, the cellshould be biased at a voltage well below the threshold voltage (Vth, orfurther abbreviated as Vt) of the “phase change” material to avoidchanging the state of the material. One method to limit the maximumvoltage allowed across a memory cell is by clamping the maximum voltageacross the “phase change” memory cell to a particular maximum valuewithin a desired read range.

FIG. 5 illustrates a schematic of a prior art sensing amplifier circuit.Sensing circuit 554 limits the voltage applied to the memory cell viathe fuse bit line (FBL) signal voltage is limited by passing the currentthrough the drain of an NMOS transistor 556, with a first referencevoltage 558 (CLAMP_REF) applied to its gate. Transistor 556 is thevoltage limiter for the fuse output 560, with the non-inverting input562 of amplifier 564 being fixed at one junction drop above the voltageof the MIL signal. Voltage CLAMP_REF 558, i.e., clamp reference, is areference voltage. The reader can appreciate the problem if informationfrom the fuse being read is needed to generate the CLAMP_REF 558 signal.

A compare reference voltage, e.g., into the inverting terminal of thecomparator 564, is set to be the desired voltage boundary between theamorphous and crystalline states of the programmable volume of the“phase change” material. The compare reference voltage signal isrepresented in FIG. 5 as the signal COMP_REF, i.e., compare reference,566.

“Phase change” memory devices use fuses for redundancy and adjusting ofreference voltages, reference currents, and timing. As one skilled inthe art will appreciate from the sensing circuit 554 shown in FIG. 5,the voltage that is being compared, i.e., COMP_REF, against the voltageacross the “phase change” memory cell is itself a reference voltage.Both reference voltages are generally programmed to a certain value withfuses. However, if the fuse used to adjust these reference voltages isalso a “phase change” memory cell, a circular situation occurs. That is,the fuses contain information for the reference voltages, but the fusesuse the reference voltages in order to be read properly.

The circular situation described above can be avoided where aself-biased first stage sense amplifier and a differential fuse strategyaccording to one or more embodiments of the present disclosure isutilized. Usage of a self-biased sense amplifier can eliminate the needfor a reference voltage to clamp the maximum voltage, where the maximumvoltage allowed across a fuse is the threshold voltage of a thin NMOS,e.g., Vt.

According to one embodiment of the present disclosure, a pair of fusesis configured in a differential manner in place of a single fuse, whicheliminates the need for a reference voltage with which to compare thevoltage across the fuse. The resistance state of the memory cell beingused as a fuse is compared against the resistance state of a memory cellthat has been programmed in a complementary fashion, thus avoiding theneed for an external reference voltage in determining the resistancestate of the fuse, and the circular situation described previously.

FIG. 6 illustrates a schematic of a self-biased sensing circuit inaccordance with one or more embodiments of the present disclosure. Theself-biased sensing amplifier 660 shown in FIG. 6 may be used as a1^(st) stage, of a fuse sense amplifier in accordance with one or moreembodiments of the present disclosure, as will be discussed further withrespect to FIG. 8. The maximum voltage across the memory cell should belimited during a read of the memory cell, such as in order to avoidcorrupting the phase of the “phase change” material, and thus thedigital data represented thereby.

The embodiment of a self-biased sensing amplifier 660 shown in FIG. 6includes a number, e.g., four, of short channel PMOS field effecttransistors (FET), e.g., 666, 667, 668, and 669, together for a longchannel (“Long L”) PMOS FET. The long channel PMOS FET is coupled inseries between Vccp, e.g., a pumped voltage source, and the self-biasedsensing amplifier 660, e.g., “SA.” The gates of the PMOS FETs are eachtied to a ground reference potential, e.g., GND. Programmable links arearranged to short across the terminals of a number of the PMOS FETs sothat the number of transistors effectively coupled in series can beprogrammed.

The drain of PMOS FET 669 is tied to the drain of an NMOS FET, e.g., M3,with the source of transistor M3 tied to the drain of another NMOS FET,e.g., M4. The gate of transistor M4 is driven by a signal READVHV 662.The source of FET M4 is connected to the memory cell sense line 664.

Two additional NMOS FETs are coupled in series, the drain of the first,e.g., M1, tied to the source of the second, M2. The source of M1 iscoupled to ground potential, and the gate of FET M1 is coupled to thesource of FET M3. Another programmable series of short channel PMOSFETs, e.g., 670, 671, 672, 673, is arranged as described previously forFETs 666, 667, 668, and 669, between Vccp and the drain of FET M2. Thegate of FET M2 is tied to Vcc. The gate of FET M3 is tied to the drainof FET M2.

According to one or more embodiments of the present disclosure, e.g., asshown in FIG. 6, the following conditions exist during a read operation.Signal READVHV 662 is driven to Vccp. The fuse bit line sense signal664, FBL, is coupled to the “phase change” memory cell (being used as afuse) through the selected access device corresponding to the particularmemory cell of interest. The PMOS devices, e.g., 666, 667, 668, 669,670, 671, 672, 673, are short channel devices (connected in series toform a long channel device) used to supply a current, e.g., a biascurrent, to the memory cell to produce a bias voltage. M1 is a biasingtransistor. The Vt of transistor M1 determines the bias level, e.g., thebias voltage, on the sense line 664, e.g., FBL.

If the voltage on the sense line 664, e.g., FBL, is near or above the Vtof transistor M1, the gate of M3 is pulled down and no pull-up currentis applied to the fuse (through sense line 664). Hence, the voltage ofthe sense line 664 on the fuse will be approximately the Vt of M1, e.g.,a NMOS Vt (˜0.35 V). The voltage of the sense line 664 can swingslightly above or below Vt, depending on the resistance of the “phasechange” material of the memory cell, e.g., +/−<100 mV, and typically+/−10 to 20 mV. If the sense line 664, e.g., FBL, voltage is below theVt of M1, then the gate of M3 is pulled up, and a pull-up current isapplied to the fuse being read, which is coupled to sense line 664,e.g., FBL, to interrogate, i.e., determine, the resistance of the “phasechange” material of the memory cell. If the sense line 664, e.g., FBL,voltage rises above the Vt, then the current applied to the fuse beingread is reduced through M3 via changes caused in the gate voltage, thusdriving the voltage back down towards Vt. In this manner the biasvoltage across the memory cell is limited to approximately the thresholdvoltage (Vt) of the bias transistor, e.g., M1. According to theabove-described method for limiting the bias voltage across the memorycell, the bias current supplied to the memory cell is used as negativefeedback to the bias transistor in order to maintain the biastransistor's junction voltage across the memory cell. The biastransistor can be configured such that the junction voltage of the biastransistor is within a read voltage region of the memory cell, therebymaintaining the voltage across the memory cell within a read voltageregion of the memory cell.

The exact value of the pull-up current is not extremely important, asvariance in the current only changes the common mode of the differentialsense line 664, e.g., FBL, signals, and only has a second order effecton the maximum voltage allowed. FET M2 is arranged as a protectiondevice that shields the thin oxide FET M1 from being presented a highvoltage as a result of the current supplied from the series of shortchannel PMOS devices, e.g., 670, 671, 672, and 673. FET M4 is alsoarranged as a protection device that shields the thin oxide FET M1 fromhaving a high voltage presented during a write operation.

According to one example computer simulation modeling the circuit shownin FIG. 6, with a break point in the programmable resistance of about 40K-ohms, the bias current is approximately steady at about 8 micro chipsbelow the resistance break point, decreasing for resistance values aboveapproximately 40 K-ohms. The output (voltage) signal from the firststage sense amplifier is flat below the 40 K-ohm break point, alsorising rapidly for larger programmed resistances. The bias voltage isclamped at approximately 0.35 V.

FIG. 7 illustrates a schematic of a 1^(st) and 2^(nd) stage of a fusesense circuit in accordance with one or more embodiments of the presentdisclosure. Two self-biased sense amplifiers 789, such as the oneillustrated in FIG. 6, are shown in simplified form on the schematic asboxes labeled “fusebias.” The embodiment of a fuse sense amplifier 780shown in FIG. 7 includes a number, e.g., four, of PMOS field effecttransistors, e.g., 781, 782, 783, and 784, coupled in series,source-to-drain, as a programmable current source between Vccp and thesource of a first control PMOS FET 786.

Buffer 788 is a level shifter, accepting inputs at Vcc voltage sourcelevels, and producing outputs shifted up in voltage to Vccp pumpedvoltage source levels. According to one or more example embodiments ofthe present disclosure, selection signal 787 is a Vcc based signal,e.g., not pumped, which is received at buffer 788. One output of buffer788 is the READVHV signal communicated to each of the self-biased senseamplifiers 789, and an inverted output driving the gate of first controlPMOS FET 786. Thus the select signal enables each of the first andsecond stages of the sense amplifier circuit 780 by making transistor M4conducting (see FIG. 6) and first control PMOS FET 786 conducting.

The gates of the PMOS FETs, e.g., 781, 782, 783 and 783, are each tiedto a ground reference potential, e.g., GND, so that they conduct andoperate as a current source. However, programmable links are arranged toshort across the terminals of a number of the PMOS FETs, e.g., 781, 782,783 and 783, so that the quantity of transistors effectively coupled inseries can be programmed into or out of the current source circuit.

In addition to the READVHV signal, the self-biased sense amplifiers 789are coupled to the memory cells bra fuse bit line (FBL) signal line,e.g., FBL and FBL′. According to one or more embodiments of the presentdisclosure, memory cells used as fuses, or storing information criticalto the operation of the memory (such as the programming of a referencesignal) may employ complimentary memory elements, for example to ensurea higher level of data integrity.

According to one or more embodiments, a pair of complementary PCRAMmemory cells comprising first and second programmable conductor memoryelements are employed, each connected to respective access transistors.During a write operation, the first and second memory elements arewritten with complementary binary values, that is: if the first memoryelement is written to a high resistance state, then the second memoryelement is written to a low resistance state; whereas if the firstmemory element is written to a low resistance state, the second memoryelement is written to a higher resistance state.

Thus, during a read operation, both the first and second programmableconductor memory elements need to be evaluated. Thus signal line FBL isused to couple the first self-biased sense amplifier 789 to the firstprogrammable conductor memory element, and signal line FBL′ is used tocouple the second self-biased sense amplifier 789 to the secondprogrammable conductor memory element containing the complementinformation.

Outputs from each of the respective first and second self-biased senseamplifiers 789, e.g., SA1 and SA2, are connected to drive the gates ofsecond control PMOS FETs 790 and 791 respectively. The drain of PMOS FET790 is coupled to the drain of a first NMOS FET 794. The source of firstNMOS FET 794 is coupled to ground. The drain of PMOS FET 791 is coupledto the drain of a second NMOS FET 795. The source of second NMOS FET 795is coupled to ground. The gates of the NMOS FETs, e.g., 794 and 795, areconnected, and coupled to the drain of second control PMOS FET 791.

The gate of a third NMOS FETs 796 is coupled to the drain of secondcontrol PMOS FET 790, with its source connected to ground and its drainconnected to the drain of PMOS FET 798. The source of PMOS FET 798 iscoupled to Vccp, and the gate of PMOS FET 798 is grounded. The output ofthe fuse sense amplifier 780, e.g., OUT, is derived from the drain ofthird NMOS FETs 796.

Because each bit of information is stored using complementary bitstorage, two memory cells are interrogated together. Thus, the output ofone of the first and second self-biased sense amplifiers 789, e.g., SA1and SA2, will be “high” and the other will be “low.” So, current fromthe current source, e.g., PMOS field effect transistors 781, 782, 783,and 784, will be sunk through one or the other of the second controlPMOS FETs 790 or 791.

NMOS FETs 796 is made to conduct (driving OUT “low”), or not conduct(driving OUT “high”), depending on whether the sense amp signal SA1,from the first stage sense amplifier circuit drives PMOS FET 790 toconduct or which of course is determined from the resistance valuestored in the memory cell, as discussed with respect to FIG. 6.

According to one example computer simulation for various common modevoltages generated by the complementary fuse arrangement shown in FIG. 7indicated robust operation of the sensing circuit for a range of inputvoltage generated by the fuse.

FIGS. 8A and 8B illustrate schematics of a memory cell configured as afuse with a “phase change” structure and an access transistor, inaccordance with one or more embodiments of the present disclosure. FIGS.8A and 8B each show a memory cell, e.g., 890A and 890B, is shown beingconfigured for use as a fuse. An access transistor, e.g., 892A and 892B,is coupled in series with a “phase change” structure, e.g., 894A and894B, having variable resistance corresponding with respective phasestates of the “phase change” structure. For example, the “phase change”structure may be a programmable volume of chalcogenide, such as thatshown in FIG. 1.

The memory cell 890A, shown in FIG. 8A, is arranged with one terminal ofthe “phase change” structure 894A coupled to the sensing line 896A,e.g., fuse bit line (“FBL”). The other terminal of the “phase change”structure 894A is connected to the source of the access transistor 892A.The drain of the access transistor 892A is coupled to ground as a sinkfor the interrogation current.

The memory cell 890B, shown in FIG. 8B, is arranged with the source ofthe access transistor 892B coupled to the sensing line 896B, e.g., FBL.The drain of the access transistor 892B is coupled to one terminal ofthe “phase change” structure 894B, with the other terminal of the “phasechange” structure 894B grounded as a sink for the interrogation current.

The access transistor for each respective memory cell, e.g., 892A and892B, is controlled with a fuse word line (“FWL”) signal, e.g., 898A and898B, to allow individual decoding for the fuse. The usage of a decodingsignal, e.g., 898A and 898B allows for a large number of fuses to besensed with a single set of biasing circuits, as shown in FIG. 7. Fusesare read in a sequential fashion, for example through the use of anoscillator, a counter, and a state machine. The results of reading eachfuse can be stored in a set of latches, as is well known (not shown).

The usage of a self-biased sense amp allows for safe sensing without thehelp of external reference voltages. The usage of a differential fusepair instead of a single ended fuse greatly simplifies sensing andprovides dramatically larger sensing margin. Each fuse pair has a fusethat is in the amorphous state being compared to a fuse in thecrystalline state, giving a factor of at least 10 between the differentresistance and a large differential signal to sense.

CONCLUSION

The present disclosure includes methods, and circuits, for operating aprogrammable resistance memory device. One method embodiment includesstoring a binary value as a resistance state in a first memory cell andas a complementary resistance state in a second memory cell. The methodfurther includes determining the resistance state of the first memorycell using a first self-biased sensing amplifier and the complementaryresistance state of the second memory cell using a second self-biasedsensing amplifier, and comparing in a differential manner an indicationof the resistance state of the first memory cell to a referenceindication of the complementary resistance state of the second memorycell to determine the binary value.

Although specific embodiments have been illustrated and describedherein, those of ordinary skill in the art will appreciate that anarrangement calculated to achieve the same results can be substitutedfor the specific embodiments shown. This disclosure is intended to coveradaptations or variations of one or more embodiments of the presentdisclosure. It is to be understood that the above description has beenmade in an illustrative fashion, and not a restrictive one. Combinationof the above embodiments, and other embodiments not specificallydescribed herein will be apparent to those of skill in the art uponreviewing the above description. The scope of the one or moreembodiments of the present disclosure includes other applications inwhich the above structures and methods are used. Therefore, the scope ofone or more embodiments of the present disclosure should be determinedwith reference to the appended claims, along with the full range ofequivalents to which such claims are entitled.

In the foregoing Detailed Description, some features are groupedtogether in a single embodiment for the purpose of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the disclosed embodiments of the presentdisclosure have to use more features than are expressly recited in eachclaim. Rather, as the following claims reflect, inventive subject matterlies in less than all features of a single disclosed embodiment. Thus,the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment.

1. A method for reading a programmable memory cell, comprising:supplying a bias current to the programmable memory cell to produce abias voltage; limiting the bias voltage across the programmable memorycell to approximately a threshold voltage (Vt) of a bias transistor;using the bias current supplied to the programmable memory cell asnegative feedback to the bias transistor to maintain a junction voltageof the bias transistor; and wherein the junction voltage of the biastransistor is within a read voltage region of the programmable memorycell.
 2. The method of claim 1, wherein the bias current is supplied toa programmable volume of the programmable memory cell, and wherein theprogrammable volume has more than one resistance state.
 3. The method ofclaim 2, wherein the read voltage region includes voltages less thanthose associated with re-programming the resistance state of theprogrammable volume.
 4. The method of claim 1, wherein the bias voltageis limited the threshold voltage (Vt) of the bias transistor plus orminus 10 mV.
 5. The method of claim 4, wherein the bias transistor is aNMOS field effect transistor.
 6. The method of claim 1, wherein usingthe bias current supplied to the programmable memory cell as negativefeedback to the bias transistor includes applying the bias voltage tothe gate of the bias transistor, bias voltage of at least Vt causing thebias transistor to turn on, the bias transistor being configured toreduce the bias current when turned-on so as to decrease the biascurrent and bias voltage.
 7. The method of claim 6, wherein applying thebias voltage to the gate of the bias transistor includes applying thebias voltage through a select transistor to the gate of the biastransistor.
 8. The method of claim 7, wherein the method includespassing the bias current through the select transistor.
 9. The method ofclaim 1, wherein using the bias current supplied to the programmablememory cell as negative feedback to the bias transistor includesapplying the bias voltage to the gate of the bias transistor, biasvoltage of less than Vt causing the bias transistor to turn off, thebias transistor being configured to increase the bias current whenturned off so as to increase the bias current and bias voltage.
 10. Themethod of claim 1, wherein the method includes: applying a positivevoltage to a gate of a transistor through which the bias current flowswhen the bias transistor is turned off; and applying a reference voltageto the gate of the transistor through which the bias current flows whenthe bias transistor is turned on.
 11. The method of claim 10, whereinthe transistor through which the bias current flows is a NMOS fieldeffect transistor.
 12. The method of claim 1, wherein the methodincludes: turning on a transistor through which the bias current flowswhen the bias transistor is turned off; and turning off a transistorthrough which the bias current flows when the bias transistor is turnedon.
 13. The method of claim 1, wherein the method includes maintainingthe bias voltage slightly above or below the Vt of the bias transistorbased on the resistance of the programmable memory cell.
 14. A methodfor reading a programmable memory cell, comprising: supplying a biascurrent to the programmable memory cell to produce a bias voltage;applying the bias voltage across a gate-source of a bias transistor;pulling a gate voltage of a transistor through which the bias currentflows to a ground potential as the bias transistor turns on therebydecreasing the bias current and the bias voltage; and pulling the gatevoltage of the transistor through which the bias current flows to asource potential as the bias transistor turns off thereby increasing thebias current and the bias voltage.
 15. The method of claim 14, whereinthe bias voltage is limited the threshold voltage (Vt) of the biastransistor plus or minus 10 mV, the Vt of the bias transistor plus orminus 10 mV being within the read voltage range of the programmablememory cell.
 16. The method of claim 14, wherein the bias transistor andthe transistor through which the bias current flows are each a NMOSfield effect transistor.
 17. A method for reading a programmable memorycell, comprising: supplying a bias current to the programmable memorycell to produce a bias voltage; decreasing the bias current and the biasvoltage in response to the bias voltage being at least a thresholdvoltage (Vt) of a bias transistor; and increasing the bias current andthe bias voltage in response to the bias voltage being less than the Vtof the bias transistor.
 18. The method of claim 17, wherein the methodincludes applying the bias voltage to a gate of the bias transistor; andwherein the bias current does not pass through the bias transistor. 19.The method of claim 17, wherein decreasing the bias current includeschanging a channel resistance of a transistor through which the biascurrent flows based on the state of the bias transistor.